The present invention relates to a process for purifying aqueous arsenic solutions containing alkali metal carbonate, sulfate and hydroxide or hydrogen carbonate, at least one metal selected from the group consisting of vanadium, uranium and molybdenum in the form of an alkali metal salt and other inorganic and/or organic impurities.
In French patent No. 2,404,601, a process is disclosed for the treatment of solutions containing sodium carbonate, sulfate, hydroxide and/or hydrogen carbonate and at least one metal selected from the group consisting of vanadium, uranium and molybdenum in the form of a sodium salt.
These solutions originate from an attack cycle of a uraniferous or vanadiferous ore, which also may contain molybdenum, after precipitation and separation of the metal which can be used in accordance with known methods.
In the case of a uraniferous ore, for example, the ore, after attack by a sodium carbonate and/or bicarbonate liquor, with the uranium solubilized and present in the liquor after the attack, was usually precipitated in the form of sodium uranate. The sodium uranate could be converted by known means--for example, either by forming crystalline ammonium uranate through sulfur-ammonium transposition upon treating the sodium uranate with a solution of ammonium sulfate, or by sulfuric acid redissolution and precipitation of uranium peroxide by the introduction of hydrogen peroxide.
Regardless of the manner of precipitation and separation of the uranium, these operations produced solutions containing sodium carbonate, sodium sulfate, and free sodium or sodium hydrogen carbonate, which had to be purified before being recycled to the ore attack step.
In addition, French Patent No. 2,404,601 discloses a process for treating the previously described solutions wherein the solutions are treated at a temperature at the most equal to boiling by a quantity of lime from 0.1 to 20 times the stoichiometric amount necessary to convert the carbonates and hydrogen carbonates present in the solution into sodium hydroxide (disregarding the quantity of lime necessary to precipitate the insoluble metallic calcium compounds). Additional steps of the process include separating and washing a first precipitate containing essentially calcium carbonate and a mixture of other organic and inorganic calcium salts, and a sodium hydroxide-enriched liquor, concentrating by evaporation a mixture of the liquor and the wash liquor of the first precipitate until a sodium hydroxide content at most equal to 50% is obtained to cause the formation of a second precipitate, separating the crystals of the second precipitate from their sodium hydroxide-enriched liquor and recovering that enriched liquor.
In that process, the second precipitate was formed essentially of sodium sulfate, which was extracted from the treatment cycle. After the sodium sulfate was removed from the treatment cycle, it could either be utilized in industrial applications or be stored in an appropriate location such as landfill zones, quarries or the like. The sodium sulfate could even be safely disposed of in rivers and streams.
However, the solutions that result from an attack cycle of vanadiferous or uraniferous ores also contain, as a particularly harmful impurity, solubilized arsenic in the form of an alkali metal arsenate, whose solubility is greater in this solution than in a solution having a lower hydroxide concentration.
Since the treatment process described in French Pat. No. 2,404,601 is applied to the above mentioned solutions containing arsenic and in order to avoid the disadvantages relating to the production of the double salt 2 Na.sub.2 SO.sub.4.Na.sub.2 CO.sub.3 during the evaporation of the liquor that results from an incomplete caustification, applicant proposed to use an amount of lime greater than the stoichiometric amount in order to precipitate the carbonate, arsenate and other organic salts in the form of a first precipitate comprising essentially calcium carbonate and a mixture or organic and inorganic calcium salts, among them arsenate.
However, in spite of its low solubility, the calcium arsenate that is present in the solid effluent formed by this first precipitate, may be an environment-polluting element and requires special and costly precautions for storage, due to the fact that the arsenate is present as a mixture with other precipitated salts. Thus, it appears desirable to selectively extract the arsenic in a form that either permits its utilization or enables easy and non-polluting storage, or even to convert the arsenate into iron arsenate which is extremely insoluble and can be stored without special precautions.
Applicant has found that it is now possible to accomplish the extraction of arsenic according to a new process that avoids the previous disadvantages.